20 



REPORT 1866. 



If the slips were of sufficient length to include the data for every class of ship, a 

 single operation would simultaneously build up Tables for all. 



A navigator wishing to find the probable diu'atiou of his intended vovage, would 

 refer to a chart on which the residts of these calculations had been protracted in 

 the form of diagrams. He must set his compasses to the radius of the diagram 

 nearest to the commencement of his intended route, measuring it in a direction 

 parallel to the route. He will thereby obtain a scale of probable distance for one 

 hour's sail dm-ing that part of his voyage, and he will prick out his passage accord- 

 ingly. When he has come ^vithin the range of another diagram he will set his 

 compasses afi-esh. Continuing on this principle, he will dot out the probable dura- 

 tion of the whole of a proposed passage in the simplest possible manner. He wiU 

 thus be able to select the quickest out of any number of routes that may be sug- 

 gested to him, and to determme, on the most trustworthy of existing data, what is 

 the best com'se to adopt in sailing from any one part of the ocean to another. 

 _ The method of altering a diagram so as to include the effect of a current, is too 

 simple to requii-e explanation. 



On the Heat attained hy the Moon under Solar Radiation. 

 Dij J. Park Harrison, M.A. 

 When the author brought forward the subject of lunar insolation a year ago, 

 be showed bya .«iniple diaoram that the Rirpliis. or accunudated heat in the moon, 

 beyond what it radiates oft' to other matter into .space, or owing to the lono--con- 

 tiuued action of the sun's rays upon her cvust_, would necessarily "i-each its maMinum 

 several days after the date of complete illumination. The mean duration of solar 

 radiation for the two periods of first and third quarters is in fact in the propor- 

 tion of 4-25 : 11 '25; and, consequently, the days on which the moon's surface 

 opposite to us IS longest withdrawn from, or exposed to, the sun's heat (in other 

 words, the days on which the moon completes her first and third quarters) would 

 be not far removed from the days of her maxinnnn and minimum temperature. He 

 has since learnt that Hen- Althaus, some few years bade, approximately estimated 

 a maximum temperature of 840° F. on the 22nd day of the lunation, seven days 

 after the day of full moon. Althaus, it appears, measured the sun's radiation 

 by the pyrheliometer, and then appWng the results to the moon, deduced from the 

 extent of her area the amount of heat intercepted ; his measure of the moon's 

 capacity for heat was that of quartz*. Assuming his deduction to be correct, the 

 heat occasionally attained by the moon would approach very closely the tempera- 

 ture at which iron appears red in twiliglit, and it exceeds the fusing-points of tin 

 and lead. Unfortunately tlie estimate cannot be compared with that made by Sir 

 John Herschel which applies to the moon's heat at the period of complete illumi- 

 nation, at which time he states that it must be far in exces.s of boiling water. But 



* Pogg. Ann. vol. sc. p. 551. 



